Method for producing binary aluminium-niobium alloys



United States Patent 3,528,806 METHOD FOR PRODUCING BINARYALUMINIUM-NIOBIUM ALLOYS Giorgio Beghi, Gavirate, and Giovanni Piatti,Varese,

Italy, assignors to European Atomic Energy Community (Euratom),Brussels, Belgium No Drawing. Filed July 10, 1967, Ser. No. 652,038Claims priority, application Italy, July 25, 1966, 17,089/66 Int. Cl.C22c 1/02, 21/00 US. Cl. 75138 4 Claims ABSTRACT OF THE DISCLOSURE Aprocess for obtaining binary aluminum-niobium alloys having improveddimensional stability under high heat conditions, rendering them usefulfor especially critical applications. The process involves producingsuch an alloy including an aluminum matrix containing a dispersion ofaluminum-niobium particles in certain specified quantity relationshipsby melting the constituents of the alloy at a temperature above 1800 C.and then casting the molten material under conditions of high speedcooling upon a body of high heat conductivity such as a thick copperbody and thereafter compacting the alloys so obtained at a temperatureof at least 580 C. In cases where a liquid is present the temperature isrestricted to not more than 750 C.

Alloys with an aluminium matrix attract great interest in the nuclearfield, particularly (but not exclusively) as the sheaths of fuelelements, mainly because of the small neutron-capture cross-section ofthis element. The compatibility with organic liquids (mixtures ofterphenyls) up to temperatures of about 450 C. is also good. Theconditions that these alloys must satisfy to be employed usefully inreactors are essentially as follows:

(a) Good mechanical properties up to a temperature of 450-500 C., so asto be able to withstand all the influences to which the sheaths aresubjected over very long periods (several years), and

(b) Good manufacturing and processing characteristics (extrusion,turning, Welding, etc.) so that the preparation of the fuel elements orother structures will not be difiicult.

The only aluminium alloy so far considered in the nuclear field for theabove purposes is SAP, made of sintered Al and A1 0 in different typeswhose A1 0 contents vary from 4% to 14% by weight. The mechanicalstrength of these alloys under heat is good, but there is thedisadvantage of an elongation that decreases with the temperature tovery low values, about 0.5% uniform elongation at 450 C. This makes itnecessary to take great precautions when designing reactors to preventthe material from being deformed. Heat-resistant aluminium alloys, whichhave better deformation capacities than SAP, cannot be consideredbecause of their very low mechanical strength at temperatures above 350C.

A more advantageous solution of the problem would therefore be theprovision of a material having a mechanical strength under heat similarto that of SAP together with a good deformation capacity, and also goodheat stability ensuring good behaviour for a long time.

The solution proposed by the invention consists essentially in a binaryaluminium-niobium alloy composed of a matrix of aluminium containing afine, uniform dispersion of particles of Nb A1 measuring about 1 micronand having a Nb percentage of up to 20% by weight in relation to thetotal of the matrix and Nb A1 particles, preferably between 5% and 12%"ice The invention also provides a process for obtaining theabove-mentioned alloy, according to which Nb A1 and A1 are meltedcompletely at a temperature above 1800 (above the melting point of Nb A1and this is followed by casting under conditions of high cooling speedand compaction under heat (at least 580-600, or in the presence ofliquid metal, but no higher than 750 C.) and extrusion if desired.

The dispersion of Nb A1 in Al is very heat-stable at temperaturesbetween about 400 C. and 500 C. for very long times of several thousandhours. The dispersion according to the invention may be produced by thefollowing successive stages:

(1) The production at a high temperature of an Al-Nb alloy containing upto 20% of Nb,

(2) Rapid solidification from a high temperature (e.g. casting of ingotsa few millimetres thick in very thick chill moulds of copper or othermetal of high heat conductivity, spraying, splat cooling), the very highcooling speed producing a structure having particles of Nb, A1 of about1 micron and even less, and

(3) The extrusion of ingots prepared with the pieces obtained as in (2).

Three examples, which do not limit the scope of the invention, will nowbe described.

EXAMPLE 1 A master alloy, consisting mainly of the compound Nb A1 wasprepared by completely melting equal parts by weight of aluminium andniobium in a helium atmosphere in an arc furnace with a tungstenelectrode. A high frequency generator was used to melt the master alloy,and aluminium was added in a quantity such that an alloy containing 10%by weight of niobium was obtained. The alloy was melted in a cylindricalgraphite crucible under argon flux at a temperature higher than 1800 C.When melting was complete, the alloy was poured into a cool cylindricalcopper mould, which had a diameter of 200 mm. and a height of 300 mm.,and in which a recess 3 mm. thick and mm. long was formed. Because ofthe large mass of copper, very rapidly cooled sheets of alloy having ahomogeneous structure were obtained. Discs and pieces were cut from theresulting castings and used, after suitable cleaning, to form an ingothaving a diameter of 54 mm. This ingot, wrapped in thin aluminium foil,was preheated for 3 hours to 580 C. and then extruded in the form of abar having a diameter of 9 mm. Tension specimens obtained from this barhad the following mechanical properties:

Temperature of test: 450 C. Uniform elongation: 2.5%. A 5 elongation:60%

Under similar conditions, specimens, having the same dimensions, of SAP(4% of A1 0 and 7% of A1 0 yielded the following values:

Uniform elongation: 0.5% A 5 elongation: 9% and 5.5% respectively.

EXAMPLE 2 The master alloy in Example 1 was used. The alloy was meltedin a horizontal graphite crucible heated by a high-frequency generatorin an atmosphere with a slight excess pressure of argon. Al and the NbA1 were put in a crucible in proportions such as to form alloyscontaining 10% of niobium; the material was melted at a temperaturehigher than 1800 C. Then, with manipulation from outside, the moltenmetal was poured onto vertically superimposed inclined copper discs witha slope such that the molten metal ran from one to the other, becomingsubdivided. The copper structure was kept cold by internal watercirculation. Solidified pellets and particles were obtained with a highcooling speed. The rest of the treatment was as in Example 1.

EXAMPLE 3 This was similar to Example 1, except that the alloy 5 wassimply poured, after being melted in a graphite crucible, onto a copperplate 20 mm. thick. The plate was horizontal but it may, if preferred,be slightly inclined.

We claim:

1. A process for obtaining a binary aluminum-niobium alloy containing NbA1 particles dispersed in an aluminum matrix and having a niobiumcontent up to 20% by weight comprising the step of completely meltingthe constituents of the alloy at a temperature above 1800 C., castingthe molten material under conditions of high cooling speed andcompacting the obtained alloy at a temperature of at least 580 C. andwhen liquid is present, at no more than 750 C.

2. The process as claimed in claim 1 wherein the high speed cooling iseffected by depositing the said molten material upon a metal body ofhigh heat conductivity characteristics, such as copper moulds, or copperplates.

3. The process as claimed in claim 2 wherein the high speed cooling iseifected by delivery of the molten metal as a spray onto the said body.

4. The process as claimed in claim 2 wherein said cooling upon said bodyis effected by the technique known as splat cooling.

References Cited UNITED STATES PATENTS 3,231,344 1/1966 Beaver et a1.75--l38 3,297,415 1/1967 Allen 75-138 3,360,350 12/1967 Sama 75138RICHARD O. DEAN, Primary Examiner US. Cl. X.R. 75.5

